The present invention generally relates to optical recording and reproducing apparatuses, and more particularly to optical recording and reproducing apparatuses for recording and reproducing an information signal on and from a rotary recording medium having a guide track which is used for guiding a light beam when recording and reproducing the information signal, by accurately controlling the tracking of the light beam.
Conventionally, when recording an information signal on a rotary recording medium (hereafter simply referred to as a disc) by a light beam, a guide track is first formed on an unrecorded disc and record the information signal is recorded on the disc by guiding the light beam by use of the guide track. The guide track may be constituted by a pair of continuous spiral tracks having a predetermined depth and located on both sides of a main track on and from which the information is recorded and reproduced. The above scheme is disclosed in a Japanese Laid-Open Patent Application No. 49-113601, for example.
When recording or reproducing the information signal on or from the disc which is formed with the pair of continuous spiral guide tracks, a pair of sub light beams for tracking are used in addition to a main light beam for recording or reproducing the information signal. The sub light beams are irradiated on the disc at positions which respectively lead and trail a position irradiated by the main light beam in the scanning direction and are mutually deviated in the width direction of the main track. A tracking control is performed so that one of the sub light beams is irradiated on a portion of one of the guide tracks and the other sub light beam is irradiated on a portion of the other guide track. Accordingly, tracking of the main track is controlled to accurately scan the main track in order to record and reproduce the information signal on or from the main track.
The above main light beam is a zero-order (0-order) diffracted light which is generated by a diffraction grating, whereas the two sub light beams are first (+1st and -1st) diffracted lights which are also generated by the diffraction grating. The optical recording and reproducing apparatus which uses these three light beams records or reproduces the information signal on or from the main track by the main light beam (the 0-order diffracted light). In addition, by use of the main light beam reflected by the disc, a focusing error signal can be detected based on an astigmatic focusing method in which a subtraction is performed between a sum of outputs of a first pair of diagonally arranged optical sensors and a sum of outputs of a second pair of diagonally arranged optical sensors so as to obtain the focussing error signal. Moreover, it is possible to carry out the tracking control of a three spot type optical head by use of the two sub light beams (.+-.1st diffracted lights).
However, the conventional optical recording and reproducing apparatus has the following disadvantages. The light intensity ratio of each of the .+-.1st diffracted lights to the 0-order light is generally 1:2 or 1:3. Therefore at the time of the recording, there is a possibility that the .+-.1st diffracted lights may damage the recording surface of the optical disc. Further, it becomes difficult to provide the 0-order light with enough power to record the information signal on the optical disc because there is a limit in the maximum output power of the laser diode. Moreover, when the light intensity of each of the .+-.-1st diffracted lights is reduced with respect to the 0-order diffracted light to prevent damage to the recording surface, no problem arises at the time of the recording. However, at the time of the reproduction, when the total light intensity of the light beam emitted by the laser diode is made considerably smaller than that at the time of the recording, the signal to noise ratio of an information signal read out by each of .+-.1st diffracted lights is degraded.